Recently, attempts are actively made to use a magnetoresistive element with a tunnel magnetoresistance effect as a storage element in a magnetic random access memory. Such a magnetic random access memory using a magnetoresistive element as a storage element can store data in a nonvolatile manner, can perform write/read operations in a period of 10 ns or less, and can perform write operations 1015 times or more.
The writing to the magnetoresistive element is usually performed by a spin transfer torque magnetization switching method, in which the magnetization of a magnetic storage layer of the magnetoresistive element is switched by injecting spin-polarized electrons (spin-polarized current) into the magnetic storage layer. Since the amount of current needed for the writing decreases as the magnetic storage layer decreases in size, this is greatly expected as a writing method for magnetic random access memories.
However, the spin transfer torque magnetization switching is basically a current-controlled writing, and at present requires a large amount of current density, about 0.5 MA/cm2. For this reason, several problems arise such as an increase in power consumption, an increase in size of periphery circuits, and an increase in area of cell selection transistors.
In order to solve these problems, recently, voltage-controlled magnetization switching writing (hereinafter also referred to as “voltage-controlled writing”) is proposed. The voltage-controlled writing applies a voltage to the magnetization storage layer of the magnetoresistive element via an insulating film to change the number of electrons in the magnetization storage layer near the interface with the insulating film, thereby switching the magnetization direction of the magnetization storage layer. The aforementioned problems in conventional spin transfer torque magnetization switching methods can be solved by this writing method. Accordingly, magnetic random access memories with lower power consumption can be produced.
However, as described above, a memory employing the voltage-controlled writing should include an insulating film on the magnetic storage layer of the magnetoresistive element. This prevents a current from flowing through the magnetoresistive element. Therefore, the resistance of the magnetoresistive element cannot be read. Thus, it is not possible to produce magnetic random access memories by simply using the aforementioned method.
As described above, a magnetic random access memory employing the voltage-controlled writing method, which is recently receiving attention, preferably has an element structure that is different from the element structure of a spin transfer torque magnetization switching magnetic random access memory, simply because a resistance value cannot be read from a structure in which an insulating film and a write electrode are disposed on a magnetoresistive element.